Sue,
Paul has a good point, in our case when mass 18 peak was higher than 2 V
the reference peak values were unstable, and optimal m/z=18 was <1V (on
the middle cup),
cheers
Dariusz
Paul Eby wrote:
> Sue,
>
> Another possibility: water. If mass 18 is high, you'll get a drift in
> results for reference peak injections, quite separate from anything
> happening in the GC.
>
> Paul
>
>
> At 09:52 AM 28/04/2006, you wrote:
>> Thanks Paul:
>>
>> I have cleaned the injector per manufacturers recommendation, but
>> omitted the brushing step since it didn't look too bad. So perhaps it
>> needs a more rigorous cleaning.
>>
>> It does seem as though the combustion column is to blame. I did get
>> two decent runs of this standard before observing mass dependent
>> fractionation. Prior to these last tests the new combustion oven was
>> oxidized at 940C for 30 minutes followed by overnight at 540C. I'll
>> go ahead and reoxidize at each temperature longer. Perhaps the
>> reaction at 940 was not complete enough.
>>
>> Thanks again for your input!
>> Sue
>>
>> On Apr 28, 2006, at 11:40 AM, Paul Eby wrote:
>>
>>> Sue,
>>>
>>> Two possibilities come to mind: your combustion oven, while new,
>>> might not have enough oxygen. As the run progresses, oxygen is
>>> depleted and results get worse.
>>>
>>> The other potential source might be your injector. If it isn't hot
>>> enough, or it's dirty, you could get a mass dependent fractionation.
>>>
>>> Paul Eby
>>> University of Victoria
>>>
>>>
>>> At 09:29 AM 28/04/2006, you wrote:
>>>> Hello All:
>>>>
>>>> I am seeking any possible hints to solve a problem that has developed
>>>> in our GC-IRMS system.
>>>>
>>>> At the moment the suite of standards we run for d13C of FAMEs in
>>>> order to check for consistency no longer exhibits a constant
>>>> correction factor across all the compounds in each standard
>>>> chromatogram which range from 14:0 through 23:0 in size. In fact we
>>>> are seeing greater depletion in 13C for the longer (or later RT)
>>>> FAMEs by as much as a few per mil relative to the first compounds
>>>> (14:0). I have been trying to isolate the variables and this is what
>>>> I've found so far:
>>>>
>>>> 1. Flow is constant throughout the entire GC run (2 mL/min).
>>>> 2. CO2 injected throughout the run remains constant in it isotopic
>>>> composition so does not exhibit the depletion see in compounds that
>>>> must be combusted.
>>>> 3. I figured it must be the combustion column so it has been replaced
>>>> and we are again seeing the same problem.
>>>> 4. Peak shape looks pretty good with widths remaining within 15-20s
>>>> and fairly sharp ratio traces. The ratio traces do become a little
>>>> broader with the last few more deplete compounds but not particularly
>>>> worse than runs where we've gotten good consistent values.
>>>>
>>>> We are using an Agilent (HP) 6890 GC with a split/splitless injector
>>>> in splitless mode. I have changed the septa (done frequently
>>>> anyway), liner, and the gold seal and washer. The GC column is very
>>>> new but also has been cut (`0.5m) to see if reactive sites may have
>>>> been a problem. We are using the metal Valco cross piece and have
>>>> been now for years without a problem, could this have developed
>>>> active sites that may yield such symptoms?
>>>>
>>>> Thanks for any information anyone might have regarding this
>>>> problem...I've been at this for some time now and am in danger of
>>>> pulling out all my hair!
>>>>
>>>> Best regards,
>>>> Sue
>>
>> Susan Ziegler
>> Associate Professor
>> Department of Biological Sciences
>> University of Arkansas
>> Fayetteville, AR 72701
>> 479.575.6944 (lab 6342)
>> 479.575.4010 (fax)
>> http://www.uark.edu/~susanz/
--
Dariusz Strapoc
Department of Geological Sciences
Indiana University, Bloomington
1001E 10th St. Bloomington, IN 47405
Tel.office (812) 856-4560
Tel.home (812) 331-0424
e-mail [log in to unmask]
|
